1. Field of the Invention
The present invention relates to a sensor for sensing analytes, and more specifically to a micro/nanomechanical sensor.
2. Description of the Related Technology
In WO2007/030240, Begley et. al. describe the use of doubly-clamped beams or membrane for the detection of chemical and biological material by generating stress induced static deformations. It relies on surface stress formation due to surface binding of biomolecules. However, the formation of this stress requires large molecules, preferably electrically-charged, to be closely-packed upon the surface so that there are repulsive/attractive interactions. As a result, there are significant limitations in the application of this technique to high-precision gas sensing which involves small, uncharged molecules at low concentrations that often do not form a uniform, dense coating through simple binding. Furthermore, the design in WO2007/030240 employs static detection technique, which relies on accurate measurement of miniscule deformations in the proposed structure. The static detection approach provides significantly weak signals even when very soft structures with high aspect ratios (i.e. geometries that are thin and long) are employed. As such, the method is highly restricted in scalability and sensitivity. Moreover, the structure is fully constructed from a flexible material (i.e. polymer), which limits the potential for integrating with silicon-based fabrication methods and actuation/readout components. Additionally, the static approach is often significantly more prone towards external perturbations (i.e. ambient vibrations, noise and drift) which limit both short and long term stability of the transduced signal, reducing the obtainable sensitivity.
In J. of Colloid and Interface Science vol. 316, pg. 687-693 (2007), Snow et. al. attempt to adapt a similar principle to gas sensing by using a cantilever single-side coated with a polymer for gas sensing. The deflection of the cantilever tip due to gas absorption-induced swelling of the polymer layer is optically transduced in a static read-out scheme. The above mentioned limitations of the static approach remain. Furthermore, the response is dependent on obtaining a large differential stress between the two cantilever surfaces, hence only one side can be coated with gas-absorbent coating to obtain optimum performance. Additionally, as with the idea of Begley et. al., the sensitivity of the device relies on the use of a high-aspect ratio (long and thin) cantilevers, which are limited in scalability and sensitivity. To improve these issues, Snow et. al. employ a complex optical readout approach, but this results in a more challenging, high-power transduction mechanism, that is difficult to integrate and multiplex into an array, particularly for low-power requiring wireless autonomous sensor nodes.
M. Li et. al. in Nature Nanotechnology, vol. 2, pg. 114 (2007) try to overcome the limitations of the static approach by employing the cantilever devices in a resonant detection circuit. A polymer coating on the cantilever is used in capturing the difluoroethane gas molecules to generate a mass-induced resonance frequency shift. However, the devices rely simply on mass effect which is often very limited for gas molecules, and detection requires the absorption of a significant number of molecules. The accumulation of the minimum detectable concentration can require a significant response time.
In U.S. Pat. No. 5,719,324, Thundat et. al. claim a cantilever (single-clamped structure) to be responsive to gaseous analytes when single-side of its surface is partially treated for formation of surface tension upon adsorption of the analyte. The vibration characteristics of cantilevers, however, are known to be largely insensitive to stress formation due to their highly flexible nature where the tip is free to deform to relieve stress through strain, and the effect is argued to be less prominent than previously claimed. Furthermore, in the proposed layout, the sensing effect is confined to the surface. As such, the proposed mechanism is highly inefficient approach in coupling the surface stress into a bulk effect in the structure. Since resonance characteristics of the device are determined by changes in its bulk properties, this approach limits resonant sensitivity of the structure. Even then, the surface stress effect is reported to be observable only within a region near the clamp. As such, this approach is highly limited in transduction area and scalability. Additionally, the setup proposed by Thundat et. al. is vibrated by a common actuation transducer which means all vibrations will be at a single frequency at any given time, limiting the possibility of characterizing vibrations from different structures in real-time. Moreover, the detection is performed using complex, high-power requiring optical means where a laser beam is aligned to the tip of each cantilever. The requirement to have individually aligned optical source and detector significantly limits both the size of each cantilever and the number of devices that can be integrated. As such, the sensor proposed in U.S. Pat. No. 5,719,324 is not suitable for gas sensing applications where selectivity can only be achieved by large arrays using differential measurement of individually sensitive devices, particularly when low-power and small form factor requirements exist as in autonomous sensor nodes.
Membranes have also been adapted to gas sensing by A. Schroth et. al. in Sensors and Actuators B, vol. 34, pg. 301-304, (1996), who employed polyimide-coated resonant membrane for detection of humidity. Significant swelling of the polyimide polymer when exposed to humidity and a frequency shift were observed. Membranes, however, due to their geometry, are significantly stiffer than doubly-clamped beams of identical length, thickness and material. Resultantly, the membranes are significantly limited in amplitudes of motion, requiring more power consuming actuation and detection schemes. As such, membrane geometry lacks scalability, requires significantly larger surface areas, and limits the ability to construct arrays of sensor devices in small form factors needed for sensor node applications. The size of the membranes also hinders their resonant operation in fluidic environments since damping effects in membranes will be more pronounced in comparison to the slender design of the beam geometry disclosed in this description. Also, the diffusion of the analyte into a membrane coating can be a slower process, as it typically provides a smaller surface to mass ratio when compared with beam geometry with identical layer thicknesses.
S. C. Jun, X. M. H. Huang, M. Manolidis, C. A. Zorman, M. Mehregany, and J. Hone, (in Nanotechnology, vol. 17, no. 5, pp. 1506-1511, 2006) describe the responsivity of composite doubly-clamped beam resonators to ambient temperature dependent thermally-induced stress effects but its unresponsive analytes of any type.
A need thus remains for sensing devices of low complexity, small size, and high sensitivity for detection of analytes, particularly those with low molecular weight.